Multi-bank memory accesses using posted writes

ABSTRACT

Systems and methods for reducing delays between successive write and read accesses in multi-bank memory devices are provided. Computer circuits modify the relative timing between addresses and data of write accesses, reducing delays between successive write and read accesses. Memory devices that interface with these computer circuits use posted write accesses to effectively return the modified relative timing to its original timing before processing the write access.

BACKGROUND OF THE INVENTION

This invention relates to read and write memory accesses in multi-bankmemory devices. In particular, this invention relates to reducing thenumber of clock cycles incurred when accessing multi-bank memorydevices.

Computers and other electronic systems usually include memorysubsystems. Typical memory subsystems include a memory controller thatcontrols communications between the CPU and various memory devices.Memory devices, such as, for example, DRAMs (dynamic random accessmemories), are widely used in computer circuits because of their largestorage capacity and relatively low power requirements. A DRAM cancontain several internal banks of memory cells, which are organized intorows and columns. Typically, a non-banked memory device is one in whichthe memory device has one memory array with one set of row-columndecoding circuitry and one set of data sensing circuitry, while amulti-banked memory device has at least two separate memory arrays, eachoperated independently with separate row-column decoding circuitry anddata sensing circuitry. One advantage of multi-banked memory devices isincreased parallelization of internal operations, which increases memorydevice throughput.

Any memory cell in a particular row of a particular bank of amulti-banked memory device can be accessed after that bank and row are“activated.” Activated banks or rows stay activated for a given periodof time, after which they are de-activated and then re-activated forfurther accesses.

During read accesses to a DRAM, there is at least a one clock cycledelay from the time a valid read address is provided to the DRAM to themoment data corresponding to that read address appears at the outputs ofthe DRAM (assuming that the bank and row accessed by that read addresshave already been activated). However, the relative timing of addressesand data for write accesses to a DRAM is different from that ofaddresses and data for read accesses. During write accesses to a DRAM, awrite address is typically provided to the DRAM at substantially thesame time as data to be written to that write address. When a writeaccess follows a read access and vice-versa, at least a one clock cycledelay results from this difference in read and write accesses. This oneclock cycle delay multiplied by the vast number of read and writeaccesses typically performed by computers and other systems with suchmemory can significantly reduce the overall speed/bandwidth performanceof those computers and systems.

In view of the foregoing, it would be desirable to reduce read-writeaccess delay and write-read access delay in multi-bank memory devices.

SUMMARY OF THE INVENTION

It is-an object of this invention to reduce read-write access delay andwrite-read access delay in multi-bank memory devices.

In accordance with the invention, computer circuits that interface withmemory devices are provided that have reduced memory access delays.These reduced delays are accomplished by increasing the relative timingbetween addresses and data for write accesses. This increased relativetiming is similar in magnitude to the delay between addresses and datafor read accesses. With this adjustment to the relative timing betweenwrite addresses and data, read accesses and write accesses can beperformed successively with reduced idle time between them.

Advantageously, memory devices are not modified to accommodate theadjusted relative timing with respect to the storage circuits that formthe memory storage areas or memory cells of the memory devices. Instead,pipeline registers are provided to re-adjust the relative timing betweenthe write addresses and write data back to the original timing or,optionally, to other relative timings acceptable to the storagecircuits. In effect, the pipeline registers contain posted writeaccesses. Moreover, incoming read accesses are monitored by the memorydevices for matches with the pending write accesses. When a matchoccurs, data is provided from the appropriate pipeline registers insteadof the storage circuits.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the invention will beapparent upon consideration of the following detailed description, takenin conjunction with the accompanying drawings, in which like referencecharacters refer to like parts throughout, and in which:

FIG. 1 is a timing diagram of known write-read accesses;

FIG. 2 is a timing diagram of write-read accesses in memory devices andcomputer circuits according to the invention;

FIG. 3 is a block diagram of a memory device according to the invention;

FIG. 4 is a block diagram of a more detailed embodiment of the memorydevice of FIG. 3 according to the invention;

FIG. 5 is a block diagram of another embodiment of a memory deviceaccording to the invention; and

FIG. 6 is a block diagram of a computer circuit according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Memory devices and computer circuits that interface with each other aretypically constructed such that addresses and data for write accessesare coincident with each other. For example, with a memory device thathas separate address and data busses, computer circuits are typicallyconfigured to present addresses and data for a write access during thesame clock cycle, or at the same clock edge. Alternatively, data ispresented at some other time convenient for the memory device (e.g.,data is presented while the memory device is decoding the column addressand is inserted into the memory array at a convenient point during thedecoding process). Within the memory device itself, addresses and datafor write accesses typically retain the same relative timing (i.e., theaddresses and data for a write access are present during the same clockcycle or clock edge).

However, addresses and data for read accesses to memory devices are notcoincident during the same clock cycle or clock edge because the data isread from the memory devices after receiving a read address, thuscreating a delay between the address and data for any is given readaccess. This delay is typically one or more clock cycles or clock edgesbecause data provided by a memory device during a read access is usuallyregistered or latched within the memory device. Thus, the relativetiming between addresses and data for read accesses is usually differentthan that for addresses and data for write accesses. Computer circuitsinterfacing with memory devices exhibiting this difference in relativetiming can experience delay (“bus turnaround delay”) between a writeaccess and a read access and vice-versa.

Timing diagram 100 of FIG. 1 illustrates typical bus turnaround delaythat occurs between read and write access cycles (or vice-versa) in amulti-bank memory device, such as an SDRAM (synchronous DRAM). Often,practical considerations require more than the one dead cycle shown inFIG. 1. This bus turnaround delay is also experienced by most singlebanked devices such as SRAM (static random access memory). The states ofthe address bus and the data bus coupled to a memory device arerepresented by address bus timing 104 and data bus timing 106,respectively. The data bus and address bus may be synchronized to oneanother, as well as to other input and output signals of the memorydevice, with a clock signal represented by clock timing 102.

For a write access from a memory controller to an address within thememory device, the address and data are typically presented by thememory controller at the same clock edge and are valid for a clockcycle. A typical write access is illustrated in FIG. 1. As shown, boththe write address and the write data for a write access are valid forclock cycle 108. If the write access at clock cycle 108 is followed by aread access to a second address (“read address”) within the memorydevice, the second address may be presented at clock cycle 110, whichimmediately follows clock cycle 108. If both read and write accessespresented during clock cycles 110 and 108 respectively fall within thesame bank (and row) such that bank and row activation does notnecessitate the use of extra clock cycles and thus, does not add to busturnaround delay, data corresponding to the read access (“read data”)may be presented as early as clock cycle 112. The read data cannot bepresented for the full period of clock cycle 110 because some finiteamount of time is required to access the portion of memory addressed bythe read address. Typically, the read data is held valid for at leastone clock cycle to ensure that it is received by the memory controller(and any other coupled devices). Thus, the data bus is idle at clockcycle 110, resulting in a “dead cycle”, and the read data is presentedduring clock cycle 112.

If memory accesses by the memory controller require totallynon-sequential addresses (i.e., accesses from different rows andcolumns), then each access requires row and column decoding andactivation in succession. In addition, if the row and column accessesinclude large numbers of alternating read and write accesses (e.g., readaccess followed by write access followed by read access), then a largenumber of additional clock cycles will be wasted because of the amountof time required to perform the read access and then turn the bus aroundto perform the write access. The invention provides dramatic improvementin this case.

If the read access is followed by a write access, the write addresscorresponding to that write access cannot be presented by the memorycontroller at clock cycle 112 because the data to be written into memorycould collide with the data corresponding to the read access. Therefore,the memory controller is likely to start the write access at clock cycle114 by presenting the write address in that clock cycle. As a result,the address bus is idle at clock cycle 112, resulting in another “deadcycle”.

Thus, the address bus and data bus are idle for one clock cycle whilethe memory device switches from one access mode (e.g., a write access)to another access mode (e.g., a read access) and back to the firstaccess mode (e.g., a write access). In some systems, the CPU mayalternate between read and write accesses frequently, thus resulting inunderutilization of the address bus and data bus as illustrated inFIG. 1. In particular, a sequence of four accesses(write-read-write-read as shown in FIG. 1 requires 6 cycles (clockcycles 108, 110, 112, 114, 116, and 118). For any sequence ofalternating read-write memory accesses, the proportion of dead cycles tothe total number of cycles needed to complete the sequence can vary froma 1:3 ratio to a 1:4 ratio.

According to the invention, the timing of the write data with respect tothe write address is advantageously modified from that shown in FIG. 1to reduce the number of dead cycles. FIG. 2 illustrates the advantageouseffect of an added delay in the timing of write data with respect to itswrite address—a reduction, if not elimination, of dead cycles. As shown,the write data corresponding to the write address at clock cycle 208 isintentionally delayed by a clock cycle such that the write data ispresented by the memory controller at clock cycle 210. If the writeaccess starting at clock cycle 208 is immediately followed by a readaccess, none of the timing aspects of the read access need be changed inaccordance with the invention. If the read access is immediatelyfollowed by a write access, that write access can start at clock cycle212, which immediately follows clock cycle 210, because thecorresponding write data presented at clock cycle 214 does not causedata bus contention with the read data presented at clock cycle 216. Amemory device that exhibits the delayed timing of write data withrespect to its write address as illustrated in FIG. 2 can operate itsaddress bus and data bus at very high efficiency, particularly for anarbitrarily lengthy sequence of alternating read-write accesses.

In accordance with the invention, memory devices can process read andwrite accesses having the timings shown in FIG. 2 without modifyingwrite access timing requirements of internal storage areas of the memorydevices. This is done by pipelining the addresses and data for writeaccesses to restore the original relative timing between the writeaddresses and data before the addresses and data are presented to theinternal storage areas. These pipeline registers delay the actualprocessing of write accesses at the internal storage areas of the memorydevices. In effect, the registers used to pipeline the addresses anddata contain “pending” or “posted” write accesses.

In accordance with the invention, read accesses from the same memorylocations for which there are pending write accesses are provided withdata from the pipeline registers instead of from those memory locations.The addresses for the pending write accesses contained in the registersare compared with addresses for incoming read accesses. If an addressstored in the pipeline registers matches the address for an incomingread access, data for that read access is provided from a pipelineregister.

Multi-bank and row-column access memory devices preferably includepipeline registers. They also include banks of memory cells that areaddressed by row and column addresses and bank select signals. During atypical read or write access to a memory location in an SDRAM, forexample, an external address bus to the SDRAM carries the row addressesand column addresses in a multiplexed fashion. The row addresses andcolumn addresses can be latched by two sets of pipeline registers inwhich one set of registers latches the row addresses and the other setlatches the column addresses. The resulting pipelined row and columnaddresses are provided to row and column address decoders, pairs ofwhich are typically provided for each bank of memory cells. Data forwrite accesses are pipelined by another set of registers that areconnected to the banks of memory cells.

If bank select signals are also required for selection of banks, thesebank select signals are also pipelined in the same manner as row andcolumn addresses. In addition, to provide read accesses to the samelocations as pending write accesses (that are stored in the pipelineregisters), row and column addresses for incoming read accesses arecompared with those stored in the pipeline registers. When there is amatch between row and column addresses for an incoming read access andthose stored in a set of row and column pipeline registers, the data forthat read access is provided by data in a pipeline register, rather thanby the memory cells. The row and column pipeline registers, datapipeline registers, bank select line pipeline registers, and all othercircuitry associated with selectively pipelining addresses and data forwrite accesses are preferably controlled by command decoder circuitryand other circuits typically used in multi-bank and row-column accessdevices. These controls detect when write accesses are occurring andcontrol the data provided to read accesses according to the invention.

FIG. 3 shows a multi-bank, row-column accessed memory in accordance withthe invention. Memory 300 includes clock logic circuit 302 and commanddecoding circuit 304. Circuits 302 and 304 generate control logic andclocks for other areas of memory 300. The address bus for memory 300 isconfigurable as either multiplexed or not multiplexed. When the addressbus is not multiplexed, row and column addresses are simultaneouslypresented to the device and latched. When multiplexed, addressinformation is presented to the memory device on two consecutive cycles.For example, the first half of the address information can be latched onthe first cycle, and the second half of the address information can belatched on the second cycle. This division of the address into twohalves is not necessarily a row-column division. The memory deviceultimately designates one portion of the address information as a rowaddress and another portion of the address information as a columnaddress. In one embodiment, address registers 308 are coupled to addressbus 307 to capture row and column addresses of write accesses to memory300. To control the latching of row and column addresses during writeaccesses, command decoding circuit 304 is coupled to address registers308. Address registers 308 are also coupled to row decoder circuits 310and column decoder circuits 320. Row decoder circuits 310 are coupled tobanks of memory cells 316 via bank select signal lines, and can access arow from a specific bank in accordance with bank select signals.

Each bank of memory cells 316 is coupled to data path control logiccircuits 318 such that specific columns from a row may be read from, orwritten to, under the control of signals from command decoding circuit304 and column decoding circuits 320. Data path control logic circuits318 control data written into the columns of data such that theappropriate data is written into memory cells 316. Data path controllogic circuits 318 also control data read from the columns of banks ofmemory cells 316 into data registers 312, which are then output ontodata bus 313.

FIG. 4 illustrates in greater detail various aspects of memory 300 inaccordance with the invention. Memory 400 achieves the delayed timing ofwrite data with respect to write addresses, described earlier and shownin timing diagram 200. Memory 400 includes row address register 402,which latches the row address from the address bus. When commanddecoding circuit 304 detects a first write command on the command bus,command decoding circuit 304 causes the address bus to be latched by rowaddress register 404. Command decoding circuit 304 is coupled toregisters 402, 404, and 406 (the connections between command decodingcircuit 304 and registers 402, 404, and 406 are not shown for clarity).

When command decoding circuit 304 detects a second write command on thecommand bus, the contents of row address register 404 are latched by rowaddress register 406. This arrangement of two row address registers 404and 406 latching the row portion of write addresses results in apipeline delay of two cycles before the row address initially latched byrow address register 402 is presented to row decoder 456, and thus tobanks of memory cells 412.

Memory 400 also includes column address registers 440, 442, and 444,which operate similarly to row address registers 402, 404, and 406.Column address registers 442 and 440 are coupled to command decodingcircuit 304 and latch the column address when command decoding circuit304 detects write commands. Command decoding circuit 304 is coupled toregisters 402, 404, and 406 (again, the connections are not shown forclarity). Although FIG. 4 shows an embodiment of the invention in whichthe row and column addresses are delayed by a minimum of two clockcycles, a memory device in accordance with the invention can bealternatively constructed such that row and column addresses are delayedby any number of clock cycles. Additionally, although FIG. 4 shows oneset of column and row address registers for delaying row and columnaddresses, other embodiments of the invention may include one set ofcolumn and row registers per bank of memory cells, such that each bankof memory cells operates independently with respect to row and columnregisters.

As illustrated in FIG. 4, data bus 427 of memory 400 is coupled to inputdata register 426, which is coupled to input data register 424. Inputdata registers 424 and 426 are also coupled to command decoding circuit304 (connections not shown for clarity). The data to be latched intoinput data registers 424 and 426 are controlled by command decodingcircuit 304 such that the contents of input data registers 424 and 426correspond to the data to be written to the row and column addressesrepresented by the contents of row address registers 404 and 406,respectively, and column address registers 442 and 440, respectively. Inone embodiment of the invention, write data is presented one clock cycleafter the write address, as illustrated in FIG. 2. If memory 400exhibits a one clock cycle delay between write addresses and write data,data from data bus 427 is latched into input data register 426 one clockcycle after the corresponding write address is latched into row addressregister 402 and column address register 444. In another embodiment ofthe invention, for example, in a DDR (Double Data Rate) memory device,the delay of data with respect to addresses for write accesses may begreater than the delay of data with respect to read addresses, ratherthan being equal in magnitude, because DDR memory devices may requireextra bus turnaround time.

The delay introduced between write addresses and write data on theaddress and data busses is removed in accordance with the invention torestore the original relative timing between the write addresses anddata for presentation at the interface of the banks of memory cells 412.Referring to FIG. 4, an introduced one clock cycle delay between writeaddresses and write data can be removed, for example, by latching thewrite address with three pairs of row and column address registers whilethe write data is latched by two registers.

Advantageously, memory devices constructed in accordance with theinvention are not limited to removal of a one clock cycle delay betweenwrite addresses and write data as illustrated in FIGS. 2 and 4. Memorydevice timing can be modified with other appropriate delays between thetiming of write addresses and write data by adjusting the ratio ofregisters latching the write address to the registers latching the writedata or by clocking the registers at only appropriate clock edges suchthat the desired timing relationship is achieved.

On a write access, the input data is delayed by two clock cycles beforebeing written into memory cells 412. In practice, a new write commandwill push a previous write access further through the write pipeline. Anew write access to the same bank in most DRAM implementations cannot beimmediately commanded due to DRAM latency limitations, although othermemory technologies may avert these limitations. Note that the inventionis not limited by the number of actual physical clock cycles. If a readaccess follows a write access to the same address and the write accesshas not yet taken place because the write address and data are in thepipeline (i.e., the write address is contained in registers 404 and 442or 406 and 444), then the data for that read access is provided by inputregisters 424 or 426.

In order to check for this condition (i.e., a read access to the sameaddress as a pending write access), adjacent pipelined row and columnaddresses are compared. In memory 400, row comparator 408 compares thecontents of row address register 402 with the contents of row addressregister 404 when register 402 contains the row address corresponding toa read access. Similarly, row comparator 410 compares the contents ofrow address register 402 and row address register 406 when register 402contains the row address corresponding to a read access.

Because addresses of memory locations in memory 400 are referenced byboth row and column addresses, the column addresses have to be providedto comparators in the same manner as the row addresses in order to checkfor the same memory address on two successive memory accesses. Columncomparator 420 is provided with the contents of column address registers444 and 442, and column comparator 422 is provided with the contents ofcolumn address registers 444 and 440. The row and column comparators areconfigured to output a signal that indicates whether the addressesprovided to the comparators are the same. For example, if the contentsof row address registers 402 and 404 are identical, row comparator 408outputs a logic-1 signal to indicate that the contents are identical.Similar output signals are provided by comparators 410, 420, and 422.Because the row and column registers have been configured under controlof command decoder 304 to correspond to the same access (e.g., rowaddress register 402 and column address register 444 contain the row andcolumn addresses for a specific memory access), logic circuit 414determines whether both row and column addresses match for comparators420 and 408 while logic circuit 416 determines whether both row andcolumn addresses match for comparators 422 and 410.

When a read access is to the same memory location as a pending writeaccess, the data for that read access is provided by input data register424 or 426 instead of from the actual memory cell. For example, if rowaddress register 402 and column address register 444 contain the sameaddress as row address register 404 and column address register 442,input register 426 provides data for the read access. Based on theoutputs of logic circuits 414 and 416, one of input registers 426 and424 provides data to data output register 428. Logic circuits 414 and416 and registers 426 and 424 are coupled to multiplexer circuit 450 forthis purpose.

If a read access does not access the same location as the two pendingwrite accesses, the read access data is provided by memory cells 412.The read access address is latched by row address register 402 andcolumn address register 440 and is selected by multiplexers 452 and 454for input into row decoders 456 and column decoders 418. Data pathcontrol logic circuit 418 controls inbound and outbound data flow to andfrom memory cells 412. When a read access is provided with data frommemory cells 412, circuit 418 drives data from a bank of memory cells.When a write access is providing data to memory cells 412, circuit 418drives data to all banks of memory cells 412.

Although FIG. 4 illustrates the implementation of posted write accessesin memories such as SDRAMs, posted write accesses can be implementedsimilarly in other types of row-column access multi-bank memories. Forexample, DDR DRAMs can use similar circuitry to reduce bus turnarounddelay with posted writes. Because data and addresses in DDR DRAMs can bevalid on both rising and falling edges of clocks, the circuitryillustrated in FIG. 4 can be modified to be responsive to both risingand falling clock edges. Alternatively, the circuitry illustrated inFIG. 4 can be modified to run at twice the DDR DRAM clock rate.

FIG. 5 illustrates one example of circuitry that can be used for a DDRmemory device in accordance with the invention. Circuitry 500 isreplicated per bank of memory cells in the memory device. Each bank ofmemory cells is enabled by bank enable signals that are output fromdecoder 530, which receives multiplexed bank enable signals from inputbank register 528. Registers 514, 516, and 518 latch the inputaddresses. Registers 508, 510, and 512 latch the input data which isinput at double the data rate with both edges of the input clock signal.Comparators 520 and 522 compare the addresses latched in addressregisters 516 and 518 so as to control whether data to output dataregister 504 is output by memory array 502 or data registers 510 or 512,so as to provide data for a read access from data registers 510 or 512,if a write access has been performed to the same address within the lasttwo write accesses. Output data register 504 provides data for readaccesses to output buffer 506.

FIG. 6 illustrates computer circuit 600 in accordance with theinvention. Computer circuit 600 includes CPU (central processing unit)602 coupled to memory controller 604 via address bus 614 and data bus616. (Note that although shown as a separate 30 block, memory controller604 can be incorporated within CPU 602.) Memory controller 604 iscoupled to memory devices (not shown for clarity) via address bus 618and data bus 620. Memory controller 604 includes memory addresstranslator 606 and memory data translator 608. Translators 606 and 608modify the formats of addresses and data transmitted by CPU 602 onaddress bus 614 and data bus 616, respectively, into formats suitablefor physical memory devices, if necessary. For example, the addressestransmitted by CPU 602 on address bus 614 may be broken down into rowand column portions by translator 606.

The timing of data output from translator 608 is preferably modified bypipeline registers 610 in accordance with timing diagram 200 of FIG. 2.Pipeline registers 610 delay the timing of data relative to addressesfor write accesses to a memory device. The number of pipeline registers610 depends on the desired modification of relative timing betweenaddresses and data for write accesses, and can be increased or decreasedto achieve desired results. For example, to introduce a one-cycle delaybetween a write address and its corresponding data, one set of pipelineregisters 610 is needed. Other amounts of delay can also be created,such as, for example, delays that are a multiple of half a clock cycle.The relative timing of addresses and data for read accesses remains thesame, and thus no pipeline registers are required to modify read accesstiming. In one approach, memory controller 604 can be configured duringinitialization of computer circuit 600 to interface with memory deviceson write accesses through programming of its configuration logiccircuits. When memory controller 604 has been configured for modifiedwrite access timing and command decoder circuits detect a write access,multiplexer 622 selects the output of pipeline register 610 to betransmitted on data bus 620. When command decoder circuits detect a readaccess, multiplexer 622 selects the output of translator 608 to beoutput on data bus 620. Addresses and data with modified timing are thenoutput on address bus 618 and data bus 620 to memory devices thatprocess modified timing of addresses and data in accordance with timingdiagram 200.

Thus it is seen that read-write access delay and write-read access delayin multi-bank memory devices can be reduced without changing the timingrequirements of internal storage circuits by using posted writes. Oneskilled in the art will appreciate is that the invention can bepracticed by other than the described embodiments, which are presentedfor purposes of illustration and not of limitation, and the invention islimited only by the claims which follow.

1-36. (canceled)
 37. A method for performing successive read and writeaccesses comprising: receiving data and an address for a write access;adding delay between said address and said data of said write accesssuch that said added delay is about equal to a delay between receipt ofan address of a read access and providing of data corresponding to saidaddress of said read access; and transmitting said write access withsaid added delay.
 38. The method of claim 37 comprising providing saidsuccessive read and write accesses to a multi-bank memory device. 39.The method of claim 37 wherein said adding delay comprises at least oneof: adding a same number of clock cycles between said address and saiddata of said write access as the number of clock cycles between saidreceipt of said address of said read access and said providing data; andadding a multiple of half a clock cycle between said address and saiddata of said write access.
 40. The method of claim 37 further comprisingremoving said added delay.
 41. The method of claim 37 further comprisingpipelining said addresses and said data of said write access to removesaid added delay.
 42. A method for performing successive read and writeaccesses comprising: adding delay between an address and data of a writeaccess such that said added delay is about equal to a delay betweenreceipt of an address of a read access and providing of datacorresponding to said address of said read access; transmitting saidwrite access with said added delay; and transmitting said read accesssuccessively after said transmitting said write access.
 43. A method forperforming successive read and write accesses comprising: receiving dataand an address for a write access; adding delay between said address andsaid data of said write access such that said added delay is about equalto a delay between receipt of an address of a read access and providingof data corresponding to said address of said read access; transmittingsaid read access; and transmitting said write access with said addeddelay successively after said transmitting said read access.
 44. Amethod for performing successive read and write accesses comprising:receiving data and an address for a write access; adding delay betweensaid address and said data of said write access such that said addeddelay is about equal to a delay between receipt of an address of a readaccess and providing of data corresponding to said address of said readaccess; transmitting said write access with said added delay; andpipelining said addresses and said data of said write address to removesaid added delay.
 45. A method for performing successive read and writeaccesses comprising: transmitting a write address of a first writeaccess in a first time period; transmitting write data of said firstwrite access in a second time period; transmitting a read address of aread access in said second time period; transmitting read data of saidread access in a third time period; and transmitting a write address ofa second write access in said third time period.
 46. The method of claim45 wherein said first, second, and third time periods are at least oneof: consecutive time periods; clock cycles; and consecutive clockcycles.
 47. The method of claim 45 further comprising presenting in asame time period said write address and said write data of said firstwrite access after said transmitting of said write address and saidwrite data of said first write access.
 48. Apparatus for performingsuccessive read and write accesses comprising: (a) means for introducinga delay between outputting an address for a write access and outputtingdata for said write access; (b) means for removing said delay betweensaid address and said data for said write access after receipt of saidaddress and said data from said means in (a); (c) means for transferringsaid address from said means in (a) to said means in (b); and (d) meansfor transferring said data from said means in (a) to said means in (b).49. Apparatus for performing successive read and write accessescomprising: means for receiving data and an address for a write access;means for adding delay between said address and said data of said writeaccess such that said added delay is about equal to a delay betweenreceipt of an address of a read access and providing of datacorresponding to said address of said read access; and means fortransmitting said write access with said added delay.
 50. Apparatus forperforming successive read and write accesses comprising: means foradding delay between an address and data of a write access such thatsaid added delay is about equal to a delay between receipt of an addressof a read access and providing of data corresponding to said address ofsaid read access; means for transmitting said write access with saidadded delay; and means for removing said added delay at said memorydevice.